I would like to draw your attention to an interesting travel report I found on an Italian scooterists' site, Cyberscooter.
Newly-wed Luigi and Maria Rita travelled across The States in a coast-to-coast trip on their Piaggio BV500.
Beside the fact that I would like to do it too, this report clearly shows that is possible to make very long trips with a relatively utilitarian scooter, with no mechanical problems.
Actually, during their group travel they write the only bikes to have had problems have been two expensive and big BMWs.....
http://www.cyberscooter.it/turismo/mondo/lettori/usa_coastocoast08ls.htm
(I am envious, envious, envious.... )